Water heaters designed for commercial, industrial and institutional applications are generally classified in one of three categories, instantaneous, semi-instantaneous and storage. A water heater falling in the instantaneous category is characterized by the absence of any significant storage capacity and by the fact that the heat exchanger occupies substantially all of the vessel or jacket. Instantaneous water heaters present difficult temperature control problems, inasmuch as cold water is delivered to the water heater and flows essentially straight through and out with no opportunity for blending and thus no opportunity for smoothing temperature variants that result from variations in demand, and the heat is supplied at a high rate, relative to the rate of water flow, thus emphasizing the lags between changes in draw and changes in heat input.
The semi-instantaneous category of water heaters is generally characterized by a relatively small tank which serves primarily as a mixing and blending zone in which water delivered from the heat exchanger is blended with water in the vessel. It is possible to obtain very close temperature control in semi-instantaneous water heaters, inasmuch as the blending principle tends to smooth out temperature gradients of water coming from the heat exchanger and thus makes the temperature of the delivered hot water less subject to variations in demand. The semi-instantaneous types often use heat anticipators for further enhancement of temperature uniformity.
The storage-type water heaters are characterized by large tanks and relatively small heat exchangers and rely upon slow rates of heating of large volumes of water in the tank to maintain the water at a desired temperature. Of the three types, the storage types have the slowest recovery, and present problems of temperature control that often cannot readily be solved, particularly in applications involving frequent large draws of hot water.
Each of the three types has various advantages and disadvantages, and the type selected is, of course, dependent upon the requirements of the particular installation. The present invention provides a solution to the problem, on the one hand, of the relatively high cost of presently known semi-instantaneous type water heaters, as compared to the instantaneous type, which results primarily from the large vessel required for presently known semi-instantaneous water heaters, and overcomes the disadvantage of relatively poor temperature control characteristic of instantaneous type water heaters. Accordingly, a water heater embodying the invention offers the size advantage of an instantaneous water heater and the temperature control advantage of a semi-instantaneous water heater.